The present invention relates to the information recording medium with particular reference to the information recording medium, with its the recording layer made up of the inorganic thin film whose optical properties are modified by the change of atomic arrangement or organic thin film which undergoes deformation, when exposed to laser beam.
When the information recording medium whose optical properties are modified by the change of atomic arrangement under laser beam is exposed to laser beam for recording, the recording layer undergoes phase change, atomic diffusion and photodarkening, resulting in change of reflectivity with respect to laser beam for reproducing; therefore, allowing information to be recorded.
The prior technique regarding phase change of the recording layer, for example, in the Japanese Patent Publication 47-26897 discloses such thin films as the Te-Ge system, As-Te-Ge system and Te-O system. The Japanese Patent Publication 54-41902 discusses the compositions of (Ge).sub.20 (T1).sub.5 (Sb).sub.5 (Se).sub.70. Furthermore, Japanese Patent Laid-Open 57-24039 describes the thin films of (Sb).sub.25 (Te).sub.12.5 (Se).sub.62.5, (Cd).sub.14 (Te).sub.14 (Se).sub.72, (Bi).sub.2 (Se).sub.3, (Sb).sub.2 (Se).sub.3, (In).sub.20 (Te)20(Se).sub.60, (Bi).sub.25 (Te).sub.12.5 (Se).sub.62.5, CuSe and (Te).sub.33 (Se).sub.67.
Moreover, the prior art on the atomic diffusion of recording layer, for example, in Japanese Patent Laid-Open 61-188752 discusses the two-layer film of (Sb).sub.2 (Se).sub.3 /Bi.
Furthermore, the information recording medium with the recording layer composed of the organic thin film which is deformed by exposure to laser beam is disclosed in the Japanese Patent Laid-pen 2-87339, Japanese Patent Laid-Open 2-87340 and Japanese Patent Laid-Open 2-87342.
In the information recording medium having a recording layer composed of the inorganic thin film whose optical properties are modified due to change in atomic arrangement when exposed to laser beam, the higher reflectivity to laser beam at the unrecorded mark or recorded mark has been conventionally set below 65 percent.
The resistivity below 65 percent, however, has been faced with the problem that direct reading of the information is not possible for the read-only CD drive, CD-ROM drive, CD-I (CD-Interactive) drive, laser disk drive and similar devices which assume the use of information recording medium having a high reflectivity.
To solve this problem, it is possible to consider increase the reflectivity to 65 percent or more at the unrecorded mark and recorded mark. But since composition to ensure higher reflectivity has not been optimized in this type of information recording medium, this method has been confronted with the problem that reproduced signal intensity is insufficient when information is recorded.
Furthermore, in the information recording medium having a recording layer composed of the organic thin film whose optical properties are modified due to change in atomic arrangement when exposed to laser beam, Au, Al, or Au and Ag--metals having a high reflectivity of 85 percent or more with respect to the laser beam irradiated for reproduction--have been used as the reflective film materials in order to increase the reflectivity of the medium. These metals have an electrical resistivity of 3.5 .mu..OMEGA..multidot.cm or less at 298K; therefore, the thermal conductivity is as large as 210 W/m.multidot.K or more, and heat is quickly removed from the portion exposed to laser beam for recording, resulting in reduced recording sensitivity. This has been accompanied by the disadvantage that the high-output type expensive as laser beam for recording has to be used.